Recently, along with highly improving density of a semiconductor element, a pattern formed by a lithography shows progress in microfabrication. The finer the pattern becomes, the smaller the requirement for size uniformity becomes. As one major cause that deteriorates the size uniformity, exposure errors of an aligner can be cited. Among the exposure errors, a focus error and an exposure amount error in particular become quite serious problems when manufacturing a device of, so-called 90 nm generation. When the focus error arises, for example, a size of an isolated pattern and the like having small depth of focus is inevitably formed small. It is said that for manufacturing a device of 90 nm generation, a focus control at a level of about ±50 nm is required.
Conventionally, the focus control in the aligner is performed in the following manner, as an example. First, a sample wafer is fabricated by adopting various focus values in on-condition maintenance and the like, the width of the isolated pattern or the like is measured using a CD-SEM or so forth, and thereafter, an optimal focus value is determined by drawing a CD-focus curve. Then, the result is stored on the side of the aligner as an offset value to thereby control the focus.
Further, as another actual example of a so-called in-line focus monitor technique, the following are disclosed. In Patent Document 1, there is disclosed a technique for measuring a focus error amount in which a relation between an angle of gradient of an edge and a focus position both of a resist pattern is obtained to thereby calculate a taper angle of the resist pattern formed on a wafer. In Patent Document 2, there is disclosed a technique for measuring a focus value in which a focus value is measured by measuring a length of a resist pattern in the longitudinal direction using a dedicated mask and a variation amount in thickness of the resist. In Patent Document 3, there is disclosed a technique for measuring a focus error amount using a reticle having a shape formed by digging a quartz substrate.
Patent Document 1: Japanese Patent Application Laid-Open No. Hei 10-154647
Patent Document 2: Japanese Patent Application Laid-Open No. 2000-133569
Patent Document 3: Japanese Patent Application Laid-Open No. 2004-184764
The focus control of the technique disclosed in the Patent Document 1 and the like requires long hours to measure a pattern size, so that the control by a time unit less than one day is difficult. Still more, it is impossible to monitor the focus variation by a time unit of several hours or less than that. In addition, in the course of a normal processing of products, no such a processing in which the focus varies in the wafer or in a lot is performed, so that a focus control using a product wafer is considered to be impossible.
Further, in the in-line focus monitor technique disclosed at present in Patent Document 2 and the like, the detectable focus variation cannot satisfy a required accuracy, causing a problem that measurement repeatability and measurement accuracy are not improved in the measurement using the CD-SEM. As a consequence, a focus monitor with high accuracy is unable to be performed.
Furthermore, in the technique disclosed in Patent Document 3 and the like, not only an algorithm for performing the focus monitor but also a process for manufacturing a reticle are complicated, so that the cost of the reticle inevitably increases significantly.